Generally speaking, there are three distinct techniques of packaging a semiconductor device, in any case said package having leads or the like exiting the package for electrically connecting the packaged die to other components, either by mounting directly to a printed circuit board or by plugging the packaged device into a socket which in turn is mounted to a printed circuit board. These are: (1) plastic molding; (2) ceramic packaging; and (3) flat packing.
U.S. Pat. No. 5,051,813 (Schneider, et al.), incorporated by reference herein, provides an example of a plastic-packaged semiconductor device. Present plastic packaging techniques involve molding a plastic "body" around a semiconductor die. Prior to molding, the die is attached to a lead frame having a plurality of leads ultimately exiting the package for connecting the semiconductor device to external circuits, such as via conductors on a printed circuit board. Various forms of plastic packs are known, including DIP (Dual In-line Package), PQFP (Plastic Quad Flat Pack) and PLCC (plastic leaded chip carder). The lead frame is formed from a single thin layer (foil) of conductive material, which is punched out to form individual leads. The inner ends of the leads are usually wire bonded to the active side (components, bond pads) of the die. When handling the lead frame, prior to the encapsulation, it is exceptionally important to avoid damaging the closely-spaced, delicate leads.
U.S. Pat. No. 4,972,253, incorporated by reference herein, provides an example of multi-layer ceramic packages which are laminated structures of alternate conducting and non-conducting layers, formed of thick conductive film and nonconductive ceramic, respectively. Generally, the conductive layers carry only one of signals, power or ground. This approach, particularly separating the signal plane (layer) from the ground and power planes, has distinct electrical advantages, which are well known. In this type of package, the conductive layers are screened or otherwise disposed between the nonconductive layers, and a very rigid, stable package is formed. For the signal-carrying layers, lead traces are typically screened onto an underlying ceramic layer. A die is eventually disposed into an opening in the package and connected to inner (exposed) ends of the lead traces. Generally, there is little problem in damaging the lead traces, since they are well supported by an underlying ceramic layer. Generally, vias are formed in the package to connect power and ground planes to particular leads in the signal plane.
U.S. Pat. No. 4,965,702, incorporated by reference herein, provides another example of a multi-layer package, using polymeric (e.g.) insulating layers and a copper foil (e.g.) for the conductive layers. Again, an object of such a multi-layer package is to provide for an electrical multi-layer conductive package which partitions (separates) the power supply system of the package from the signal transmission system as much as practical in order to optimize the performance of both.
These two multi-layer ceramic and polymer packages are also known as "chip carriers". Both are preferably completely formed prior to mounting the semiconductor die within an opening in the chip carrier, and in both the inner leads are well-supported. Hence, both of these chip carriers inherently avoid the problem of lead damage during handling and mounting of the die.
FIGS. 1A and 1B show an example of tape-based flat packing. As illustrated herein, a semiconductor device assembly 10 includes an upper, segmented plastic film layer 14 (formed of segments 14A, 14B, 14C and 14D), a lower plastic film layer 16, metallic leads 18 sandwiched between the two plastic layers 14 and 16, a metallic (preferably copper) die attach pad 20 supported between the two plastic layers 14 and 16, a semiconductor device 22 mounted on the die attach pad 20 and bond leads 24 connecting the semiconductor device 22 to the leads 18. It is also known to employ conductive "bumps" on the inner ends of the leads, rather than bond wires, to connect the leads to the semiconductor die 22, in a tape automated bonding (TAB) process. The upper and lower plastic layers are suitably formed of polyimide, and form a thin, insulating supportive structure for the leads 18. A square, insulating ring ("body frame" or "dam") 26 is disposed atop the leads 18 between portions 14B and 14C of the upper plastic fill layer, outside the die area. A layer-like quantity of silicone gel 28 is disposed over the die 22 and bond wires 24, and acts as an ionic contamination barrier for the die and as a stress relief for the leads 24 during assembly of the semiconductor device assembly, and further prevents an ultimate encapsulation epoxy 30 from contacting the semiconductor die. Evidently, the inner ends of the leads 18 are very fragile, and extreme care must be exercised when assembling the die 22 to the leads 18. In this respect, tape mounting a semiconductor die requires a similar degree of extreme care when mounting the die to the fine-pitch conductive leads.
Further examples of mounting semiconductor devices to a tape structure are shown in U.S. Pat. Nos. 4,800,419 and 4,771,330, incorporated by reference herein.
As used herein, the term "semiconductor device" refers to a silicon chip or die containing circuitry and bond sites on one face, and the term "semiconductor device assembly" refers to the semiconductor chip and associated packaging containing the chip, including external package leads or pins for connecting the semiconductor device assembly to a socket or a circuit board, and including internal connections (such as bond wires, TAB, or the like) of the chip to inner ends of the leads.
The aforementioned patents relate to semiconductor device assemblies having a high lead count, which is "de rigueur" in modern semiconductor devices. The plastic packaging and tape mounting techniques are generally indicative of methods of mounting semiconductor devices to preformed lead frames having a plurality of extremely delicate conductors connecting to the die.
As mentioned above, there are generally two techniques for connecting a die to inner ends of lead frame conductors, namely wire bonding and tape-automated bonding (TAB). In TAB, "bumps," typically formed of gold, are located on either the die ("bumped die") or on the inner ends of the lead fingers ("bumped tape"). See, e.g., U.S. Pat. No. 4,842,662, FIGS. 5 and 6, respectively.
U.S. Pat. No. 4,842,662, incorporated by reference herein, discloses bonding integrated circuit components directly to a TAB tape, without the intermediary of a gold bump, by use of a process employing ultrasonic energy, pressure, time, heat and relative dimensions of the TAB tape. Generally, the end of a lead is "downset" (urged down) onto a die. (See column 6, lines 5-8). This may be thought of as a "bumpless" TAB process.
While the above-referenced patents teach various techniques of forming lead frames, TAB tapes, and the like, and various techniques for connecting semiconductor dies to same, these techniques generally involve only one layer, or plane, of patterned metal conductors (lead fingers), which single conductive layer represents a single plane carrying signals, power and ground to the semiconductor die.
As mentioned hereinabove, it is electrically desirable to provide distinct planes for carrying signal, power and ground from leads (or pins) exiting the package to the die within the package.
U.S. Pat. No. 4,933,741, incorporated by reference herein, discloses a multi-layer package for integrated circuits having a ground plane (20) electrically isolated from a plane of conductors (14) by means of an insulating layer (16) formed of polyimide. The ground plane (20) is connected to selected conductors (14) by means of vias (18) extending through the insulating layer (16). The remaining (non-grounded conductors) carry signals and power to/from the integrated circuit device (11). As pointed out therein, "[b]ecause of the small physical size of the electrical conductors 14, they represent a significant impedance to operating potential and current 15 applied to the integrated circuit 11 causing an undesirable voltage drop along the length of the conductors 14. Additionally, capacitive coupling between the conductors 14 causes cross talk on the conductors 14 which apply signals to and/or derive signals from the integrated circuit 11. Further, the impedance of the conductors 14 create switching noise when the DC operating current 15 applied to the integrated circuit varies." And, as noted therein, "the capacitive cross coupling between the conductors 14 can be reduced by a [separate] ground plane 20 which also surrounds the integrated circuit 11 and is located adjacent the plurality of conductors." (See, especially, column 2, lines 31-46).
Despite the generally accepted notion that providing a separate ground plane has desirable electrical characteristics, the examples set forth above are limited to rigid, multi-layer ceramic or polyimide or polymer chip carriers. In both of these multi-layer approaches, it is relatively feasible to provide vias between separate metal layers and the intervening insulating layers.
On the other hand, in a tape-mounted, flexible substrate, semiconductor device assembly, it has generally not been very practical to consider or implement incorporating a distinct ground plane, since this type of "flexible" packaging does not lend itself readily to such a multi-layer approach employing vias spanning insulating layers.
For example, commonly-owned, co-pending U.S. patent application Ser. No. 07/829,977, entitled RIGID BACKPLANE FOR A SEMICONDUCTOR DEVICE ASSEMBLY, filed on Jan. 31, 1992, by Michael D. Rostoker, discloses an integrated circuit device package (semiconductor device assembly) having a flexible substrate including an upper patterned insulative layer, and a lower patterned conductive layer including a plurality of package leads (lead fingers). The assembly further includes a rigid or semi-rigid lower protective layer, formed of ceramic, glass, metal, plastic, and combinations thereof, which provides enhanced protection from mechanical and electrical degradation of the packaged device, and which may also serve as a heat sink. Thus we see that even though it is contemplated to have a rigid lower layer, which may be metal (i.e., electrically conductive), it is not contemplated to use the rigid lower layer as a ground plane connecting electrically to the die. (This is to be distinguished from the possibility that the rigid lower layer could be grounded to provide some shielding, but not connected within the package to the die.) The disclosure of this application is non-essential material.
Prior to packaging IC dies or other semiconductor devices in accordance with the above-described techniques, it is often desirable to test individual dies before separating them from a semiconductor wafer. On-wafer testing significantly improves manufacturing efficiency and product quality by detecting IC defects at the earliest possible stages in the manufacturing and assembly process. Wafer probe cards are commonly used to perform this type of in-process IC testing. A wafer probe card typically includes a printed circuit board and a number of probe leads capable of making electrical contact with power, ground and signal bond sites or test pads on the IC surface. The printed circuit board includes a pattern of traces which make electrical contact with the probe leads to carry signals or voltages from external test equipment through the probe leads to the IC. Individual ICs can thus be powered up and tested without separating them from the wafer.
One known technique of wafer probe card construction uses manual alignment and attachment of discrete probe leads to the printed circuit board. The probe leads are usually long, thin and constructed of either tungsten or beryllium-copper alloys. Each IC probe is typically individually soldered to the printed circuit board. The soldering process is particularly difficult in the case of complex ICs given the large number of probe leads which must be attached to the printed circuit board within a limited area. Manual wafer probe card assembly under current practice is therefore time-consuming and expensive. Manual assembly limits the density of probe leads which can be accurately aligned and soldered to about 300 probe leads per printed circuit board. A center-to-center spacing between adjacent probe leads is generally referred to as "pitch". Manual assembly also requires that adjacent probe leads have a pitch of greater than about 80 micrometers.
FIG. 8A shows a side sectional view of an exemplary prior art wafer probe card assembly. A wafer probe card shown generally at 800 includes a printed circuit board 812 with an inner peripheral edge 813 defining a central opening. An anodized aluminum insulating ring 814 is disposed within the central opening 813. Wafer probe card 800 is used to test an IC die 818 formed on a semiconductor wafer 817. The portion of the wafer including the die 818 under test is positioned under the insulating ring 814 and central opening 813. A number of wafer probe leads 820 are manually soldered at an outer end 828 to printed circuit traces 821 formed on a lower surface 822 of printed circuit board 812. A via hole 823 plated through with a conductive material connects each lower surface trace 821 to a respective overlying upper surface trace 824. The probe leads 820 are secured to insulating ring 814 with epoxy 825. Epoxy 825 holds the probe leads in place a suitable distance apart such that a probe tip of each probe lead is properly positioned for contacting IC die 818. An external tester board 826 includes a number of pins 827 for making electrical contact with upper traces 824 along the outer periphery of printed circuit board 812. The external tester board 826 applies appropriate test signals to probe leads 820 by plated through via holes 823 and lower surface traces 821 and thereby to IC die 818.
In order to test a typical IC, a relatively large number of probe leads 820 are required to interface with the die 818. As the complexity of the IC increases the required number of probe leads also increases. Each of the probe leads 820 must be connected to traces 821 in order to interface the IC to external test equipment. Since the connection typically involves manual soldering it can be seen that the difficulty of connecting probe leads 820 with traces 821 is a function of the density and pitch of the probe leads required on the wafer probe card to test a given IC.
A number of techniques have been developed which use TAB tape or other types of flexible substrates with etched leads to avoid the problems associated with the manual alignment and assembly process described above. U.S. Pat. No. 5,189,363 discloses a technique for improving probe lead density and pitch by using etched TAB tape leads as wafer probe leads. The TAB tape leads are brought into contact with the die under test by applying downward pressure to the inner end of the leads with a pressure anvil. U.S. Pat. No. 5,036,380 discloses a technique of using TAB tape etched leads for die probing and connecting certain of the etched leads to bum-in testing pads on the tape to provide improved IC burn-in testing. U.S. Pat. No. 4,968,589 discloses a probe card having a number of etched leads on a dielectric substrate. A ground plane is formed on the opposite surface of the substrate. The etched leads are used as probes to connect pads on an IC chip to leads on a printed circuit board.
None of the above techniques includes a suitable means for attaching certain of the etched leads to a ground plane layer. Attachment is therefore normally performed in a manner similar to that shown in FIGS. 2A and 2B of the present invention. U.S. Pat. No. 5,036,380 specifically provides that "the ground plane is connected to the ground conductors on the first surface by way of vias formed through the TAB tape." See column 3, lines 41-44. Furthermore, none of the TAB tapes presently used for wafer probe cards include an efficient means for attaching certain probe leads to one of several metallized reference planes. A wafer probe card incorporating presently available TAB tapes such as these will therefore suffer from drawbacks similar to those discussed above and in conjunction with FIGS. 1A, 1B, 2A and 2B in the context of IC packages.
The above described techniques for utilizing TAB tape in wafer probe cards suffer from additional drawbacks. These techniques generally involve using an etched lead to probe an IC die. In many cases the etched lead material is not optimal for wafer probing. When a TAB tape is used in a packaging application, the desirable physical properties of the etched lead material include tensile strength and elongation. In a typical TAB tape these properties are optimized for bonding to the IC die. When the TAB tape is used for wafer probing, a different set of properties are desirable for the probe tips of the probe leads in order to provide repetitive contacts to the IC bond sites with adequate lead deflection and form retention. These properties include stiffness and hardness of the probe tip. The probe tip should be designed such that it deflects under pressure during probing and yet is able to return to its original position when pressure is removed. The probe tip should also maintain its formed shape despite repeated probing. It may be difficult to meet these conflicting requirements in many applications by simply using the end of an etched lead. U.S. Pat. No. 5,189,363 suggests forming or coining the ends of the etched leads into various shapes. See column 8, lines 46-54. The use of alternative materials for the etched leads are also suggested. See column 8, line 55 to column 9, line 42. These suggestions all involve using the same material for the entire etched lead. It is therefore not possible to optimize the probing properties of the probe tip end of the lead while simultaneously optimizing the desirable properties of those portions of the probe lead which do not contact the die. The TAB tape within the wafer probe card will therefore have to be replaced frequently, resulting in additional expense and delay.
Presently available wafer probe cards therefore limit the efficiency and capacity of IC testing. The density and pitch of wafer probe leads is unduly restricted since the TAB tape used in the wafer probe card presently requires vias or other inefficient means of interconnecting specific probe leads to one or more reference planes. Connections to ground or power planes will continue to take up excessive space on the tape which could otherwise be used to provide more signal line interconnections to the IC. It may therefore not be possible to detect a desirable level of circuit defects in a particular IC using present techniques. Furthermore, the probe tip portion of the TAB tape probe lead wears out frequently thereby requiring replacement of the TAB tape and resulting in additional delay and expense. The limited test capabilities of existing wafer probe cards will become an even greater problem as more complex and densely packed ICs are developed.
Hence, we see that there are various desirable and unfulfilled objectives in the design and implementation of tape-mounted, flexible-substrate semiconductor device assemblies. It can also be seen from the foregoing that there is a need for an improved wafer probe card with a higher density of fine pitch probe leads.